1. Field of the Invention
The present invention relates to a heat resistant plastic film and a gas barrier film having super high gas barrier property using the heat resistant plastic film as a substrate. More specifically, it relates to a gas barrier film suitable to substrates for various kinds of devices and coating of the substrate and having super gas barrier property. Further, the invention also relates to an image display device excellent in durability and flexibility obtained by using the gas barrier film, particularly, an organic electroluminescence device (referred to hereinafter as organic EL device).
2. Description of the Related Art
Gas barrier films in which thin films of metal oxides such as aluminum oxides, magnesium oxides, and silicon oxides are formed on the surfaces of plastic substrates or films have been generally used so far for packaging of articles requiring shielding from steams or various gases such as oxygen, or packaging application for preventing denaturation of foodstuffs, industrial products, and medicines. Further, the gas barrier films have also been started for use in substrates of liquid crystal display devices, solar cells or EL devices in addition to the packaging use. Particularly, in transparent substrates which have been applied progressively to liquid crystal display devices, EL devices, etc., high level of demands such as long time reliability, high degree of freedom in view of shape, and capability of display on a curved surface have been required in addition to the demand for reduction in the weight and increase in the size in recent years, and film substrates such as made of transparent plastics have been started for use instead of glass substrates, which are heavy, tend to be cracked and are difficult in increasing the area. Further, since the plastic substrates not only can cope with the requirements described above but also can be applied to the roll-to-roll system, they are advantageous over glass materials in view of the productivity and the reduction of cost.
However, film substrates such as of transparent plastics involve a problem of poor gas barrier property in comparison with glass. Since steam or air permeates in a case of using a substrate of poor gas barrier property, when it is used, for example, to a liquid crystal display device, liquid crystals in a liquid crystal cell are deteriorated and deteriorated portions result in display defects to deteriorate the display quality. For solving such problems, it has been known to form a thin film of a metal oxide on the film substrate to obtain a gas barrier film substrate. As gas barrier films used for packaging materials and liquid crystal display devices, those formed by vapor depositing silicon oxide on a plastic film (for example, in JP-B No. 53-12953 and those formed by vapor depositing aluminum oxide (for example, in JP-A No. 58-217344) are known and they have a steam barrier property of about 1 g/m2/day.
However, highly barrier performance has been demanded recently for the film substrate along with increase in the size of liquid crystal displays or development for highly fine displays. Further, most recently, development has been progressed, for example, in organic EL displays or highly fine color liquid crystal displays requiring further higher barrier property and it has been demanded for a substrate having a performance of higher barrier property, particularly, a steam barrier property of less than 0.1 g/m2/day while maintaining transparency usable therein. For coping with such a demand, film formation by way of a sputtering method or a CVD method of forming a thin film using plasmas generated by glow discharge under low pressure conditions has been studied as means capable of expecting highly barrier performance. Further, it has been also proposed a technique of preparing a barrier film having an alternate laminate structure of organic layer/inorganic layer by a vacuum vapor deposition method (for example, refer to U.S. Pat. No. 6,413,645 and “Thin Solid films” written by Affinito, et al., (1996), p. 290 to p 291.
However, in the thin film forming methods described above, since organic materials jetted out as vapors at high temperature are condensed to form thin membranes on the film, the film is temporarily heated to partially cause deformation thereby mating the subsequent lamination step not uniform to result in a problem that no sufficient barrier performance can be obtained. In addition, since a thin layer structure is necessary in order to obtain a compatibility with the flexibility, this also gives rise to a problem that the barrier performance is impaired even when obstacles are present even in a small amount.
It has also been proposed a technique of improving the surface shape of the film substrate thereby obtaining high barrier performance. However, in a case of actually using the thus barrier film described above applying a treatment at high temperature at least of 250° C. or higher such as for forming TFT, the barrier performance of the film is deteriorated by the deformation of the substrate (JP-A Nos. 2002-225169 and 2003-154596).